The northern smooth shore crab, Cyclograpsus insularum, is a marine large-eyed crab of the family Varunidae (formerly in Grapsidae), found around Lord Howe Island, Norfolk Island, Kermadec Islands and in New Zealand from North Cape to East Cape. Their carapace width is up to 22 mm.
This species is easily confused with the smooth shore crab, Cyclograpsus lavauxi, which has a wider range in New Zealand, but similar habitat and eating habits. The northern smooth shore crab is smaller and has a smoothly graded colour from brown/purple at the front, to paler brown towards the back, whereas C. lavauxi is distinctly speckled. The righthand claw of the northern smooth shore crab shows a distinctive gape between fingers. It is found just below the high tide level on exposed shores in the presence of boulders.
Breeding takes place in winter from June to August, and there is only one brood. Eggs are relatively large, measuring 0.45 mm across and they are dark purple. They hatch after 7 weeks.